Use required keyword

[bugproof]

Request classes in this library sometimes have many required properties and it's ugly to initialize them all using constructor e.g. https://github.com/bugproof/BaseLinkerApi/blob/master/BaseLinkerApi/Requests/ProductCatalog/AddInventoryProduct.cs

dotnet/csharplang#3630

[bugproof]

This could already be fixed but requires huge amount of testing to really know what's required and what's not